We have designed and built an ultrahigh energy defibrillator using two 18,000 joule capacitor banks for the study of the defibrillatory energy requirements of large experimental subjects. One of the banks can be patched to deliver up to 800, 1600, or 2400 volts: the other bank can be patched for up to 5,000, 10,000, or 15,000 volts. The capacitor banks are electronically switched with SCRs or hydrogen thyratrons to deliver rectangular, trapezoidal, truncated exponential, or untruncated exponential waveforms. We have completed studies of the effectiveness of single-pulse rectangular wave and truncated and untruncated exponential waves in reversing ventricular fibrillation in 90-110 kg calves. In the proposed work we plan to extend this work to cover multipulse rectangular wave shocks. We anticipate using pulse amplitudes of 50 through 140 amperes, pulse widths of 8 through 0.5 msec, and pulse separations of 3, 6, and 12 seconds. The data which this study is designed to generate should be useful in helping to resolve questions concerning the most appropriate technique for defibrillating human patients.